In linguistics, the unidirectionality hypothesis proposes that grammaticalisation works in a single direction.
The hypothesis is not universally applicable, with some rare counterexamples appearing in unusual circumstances.
One counter-example is the evolution of a new pronoun for "we" out of verbal conjugations in northern dialects of Irish Gaelic.
A similar change has taken place in French, where the loss of most of the verbal endings (in the spoken language at least) has meant that subject pronouns are now required.
The unidirectionality hypothesis would predict that this paradigm would either remain as it is, with the pronouns retaining their status as independent words, or else that they might fuse with the verb into a new verbal conjugational system, as existed in Old Irish.